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Is Time Travel Possible?

Time is "resistant to simple definition," according to renowned astronomer Carl Sagan. He’s quite right: Many of us think we know what time is, but we would have a hard time actually defining it. It’s not a tangible thing that you can literally see or touch, but its effects are evident as we look at ourselves aging or trees growing. Time also seems to propel us in certain directions – whether to meet a deadline or catch a plane on time – based on the numbers on your clock.

Scientists, however, often describe time as the fourth dimension of our universe. (The other three dimensions pertain to space, including up, down, left, right, backward and forward.) The theory is that time cannot exist without space, and likewise, space cannot exist without time. This interconnected relationship of time and space is called the space-time continuum, which means that any event that occurs in the universe has to involve both space and time.

Albert Einstein had a theory of special relativity, wherein time slows down as an object approaches the speed of light. Today, this theory leads many scientists to believe that traveling faster than the speed of light could open up the possibility of time travel to the past as well as to the future.

Right now, the speed of light is believed to be the highest speed at which something can travel, so modifying our DeLoreans, and traveling "Back to the Future" is unlikely. As an object nears the speed of light, its relativistic mass increases until, at the speed of light, it becomes infinite. Accelerating an infinite mass any faster than that is impossible, or at least it seems to be right now.

However, there are other theories that lead to the notion that space travel will be a thing of the future.

Wormholes: Also known as Einstein-Rosen Bridges, these could allow travel many light years from Earth in a fraction of the time it would take with conventional space travel.

Cosmic strings: Thinner than an atom, these create intense gravitational pull on objects near them. Because their force distorts spacetime, some wonder if they can be used for time travel.

Black holes: When stars reach the ends of their lives, they create implosions known as black holes. Their gravitational fields are so strong that light can’t escape and anything coming in contact with it will be sucked in. Kerr holes — a type of black hole — have rotating black holes that some say could be used for time travel portals or to travel to parallel universes.